This invention relates to tap changers for electrical inductive equipment. Tap changing is used extensively in a wide variety of electrical inductive apparatus such as AC voltage regulating transformers, HVDC rectifier and inverter transformers and phase angle regulators to adjust the devices turns ratio or phase angle while the-device is serving load. Most of the tap changing methods in use utilize a switching means to alternately connect various sections of one winding of the electrical inductive apparatus into a circuit. One extensively used switching means is a mechanical contact switch in which a movable contact selectively engages stationary contacts connected to various sections of the winding so as to connect varying numbers of turns into the circuit. This methodology is at present used to the virtual exclusion of all other methods in large power apparatus.
Another type of switching means, which has been the subject of patents activity and technical papers with increasing frequency due to its fast response time and lack of mechanical wear, is the electronic switch. Electronic switches are typically electronically controlled gate devices, such as thyristors and gate turn off devices, which are connected as an inverse parallel-connected pair to each tap of a winding, as shown in U.S. Pat. No. 3,195,038. A control device triggers predetermined groups of the thyristor pairs to connect or bypass certain ones of the tap winding sections and thereby provide a range of individual output voltage increments for an electrical inductive apparatus. The total number of discrete voltage increments may be extended, as shown in the above referenced patent, by utilizing additional thyristor pairs with each winding section to reconfigure the relative polarity of the winding sections and thereby dispose each winding section in additive or opposing relation with the remainder of the winding sections. Further, tapped secondary windings may be utilized with appropriate switching devices to increase the tap range of the electrical inductive apparatus as shown in U.S. Pat. Nos. 3,195,038, 3,909,697, and also 3,700,925.
Several of the prime considerations affecting economical usage of solid-state tap changers in electrical inductive apparatus are the total number of electrical switches, their individual ratings, their associated losses, and the number of individual winding sections within the inductive device required to provide a predetermined number of output voltage increments within the total output voltage range of the electrical inductive apparatus. Obviously, the cost of the tap changer mechanism may be reduced if fewer thyristor switches of lower rating are use with a simpler winding configuration. Prior art electronic tap changing arrangements have certain drawbacks regarding these considerations since they require an excessive number of switches and individual winding sections to provide a large number of discrete output voltage increments required in commercial applications of such electrical inductive apparatus. Due to the extreme cost none of the proposed arrangements for controllable electronic tap changers were at any time placed into commercial operation in large power equipment.
Efforts have been made to address this uneconomical condition with the use of phase control, as taught in U.S. Pat. No. 4,220,911, but this configuration produces unacceptably high levels of harmonics and as such is unacceptable practically since filters or other methods of mitigating the effect of these parasitic harmonics must be used, Other approaches, such as those described in U.S. Pat. Nos. 4,135,126, 3,786,337, 3,319,153, 3,600,664, 3,684,949, and 3,461,378 have likewise not proved satisfactory. At present there is still no commercially viable arrangement, of any form, that makes the solid-state tap changer economically attractive for commercial use.
Thus, it is desirable to provide an electrical inductive apparatus which has an improved tap changing means which provides a commercially acceptable number of output voltage increments through the use of a reduced number of individual switch devices having lower ratings, a simpler and less costly tap winding structure, and a control logic which makes the most efficient use of the ratings of the electronic components.
An objective of the present invention is to provide a completely non mechanical contact switching device having a high speed of response and high reliability, as well as economical cost. Another objective of the invention is to provide reliable operation and switching independent of the nature of the load at any moment in time. Another objective of the invention is to provide reliable switching action between any two tap settings including switching taps sequentially or selectively over the entire tap range. It is also the objective of this invention to design the control so that the switch can be operated such that the output RMS voltage is controllable and selectable between the distinct voltage increments dictated by the winding configuration. An additional objective of this invention is to use the switch as a device to limit the duration of the fault current through the apparatus and the switch itself so that the device rating and cost of the device be reduced significantly.